In a world of constantly increasing rate of information transmission, reduction of transmission cost and increased efficiency are important. Notwithstanding the advantages of optical information (i.e. the speed of the information transmission and low manufacturing cost), optical transmission is still not as dominant as the electrical information transmission through the communication world. A key reason for the continued use of electrical over optical communication is the loss of energy at the coupling between the optical devices (such as diodes, optical fibers, and waveguides).
Coupling between optical devices, such as between optical fibers (simply referred to in the field as fibers) and waveguides, is a difficult problem in the sense of the obtainable energetic efficiency, in other words reducing loss of signal strength between coupled products. One conventional technique used to couple between optical devices is coupling facet to facet, for example, where a facet of a waveguide is coupled directly with a facet of a fiber. Referring to FIG. 1A, a diagram of coupling using a funnel, a waveguide 102 with width. W has a funnel coupler 104 (simply referred to in this document as a “funnel”). Note, the transition from the waveguide 102 to the funnel 104 is shown as funnel base 108, a construction line used for reference purposes. The facet of the funnel used for coupling to another optical device, such as fiber 106 is shown as funnel facet 109, and is also referred to in industry as the “coupling area”. The funnel shaped coupling area has the advantage of a large coupling area at funnel facet 109 to receive a transmission from fiber 106. The funnel 104 and corresponding funnel facet 109 can be made as big as necessary for the particular application. This technique of using a funnel has disadvantages, including a high loss of efficiency due to mode-size and effective index mismatch at the funnel facet 109 of the funnel coupler 104 and the fiber 106.
Referring to FIG. 1B, a diagram of coupling using tapering, a waveguide 102 with width W includes a tapered coupler 114. Similar to the above description of using a funnel, the transition from the waveguide area 102 to the tapered coupler 114 is shown as taper base 118. The facet of the taper used for coupling is shown as taper facet 119, the coupling area for tapered coupler 114. The coupler is typically made as part of the waveguide 102, of high-index contrast material, and may be connected with a short taper with a nanometer-sized tip. Typically, nanometer-sized tips are on waveguides. Tapered tips of fibers are in the range of 0.5-5 μm in width. Tapers for diodes are typically on the same scale as fibers. The taper shaped coupling area has advantages including being more energetically efficient than the funnel technique, and of effectively doing the required mode conversion. This technique of using a taper has disadvantages, including a small coupling area (the nanometer-sized tip) which is difficult to couple to fiber 116, and the small tip that can easily be damaged.
A typical application is to couple waveguide 102 to a single-mode optical fiber (106, 116). Waveguide 102 is typically fabricated on an integrated circuit (chip) and is coupled “off-chip” to other optical devices, such as optical fibers (106, 116). The fiber (106, 116) is typically tapered from being the width of the fiber (portions 106A, 116A, for example core dimensions of 8 to 10.5 μm in width) to a smaller diameter (tapered portions 106B, 116B, for example about 3 μm in width) as appropriate for the application and specific coupler (104, 114) being used.
High refractive index material allows the fabrication of sub-micrometer-sized structures such as waveguides. Coupling to and from devices such as waveguides, fibers, diodes, and optical switches, usually involves high losses due to mode-size and effective index mismatch, for example, between an optical fiber and the waveguide structure, which induces coupling to radiation modes and back-reflection. The fiber (106, 116) typically has a tapered edge such that at the tip the field distribution matches better to the mode field profile obtained at the edge of the nanometer waveguide tip (becomes the field at the edge of the coupler tip delocalized from the waveguide core). The delocalization of the mode field profile at the edge of the nano-tip increases the mode overlap with the optical fiber mode.
An example waveguide has height H=220 nm (the height of the silicon on the silicon on insulator (SOL)) and width W=450 nm, in order to achieve a single-mode operation. To convert the low-confined local mode at the nano-taper tip into the high-confined waveguide mode, a short tapered transition is employed by gradually varying both sidewalk in a symmetric parabolic transition towards the final waveguide width. The coupler losses are ultimately governed by the mode mismatch loss between the mode at the nano-taper tip facet and the one at the edge of the tapered fiber.
There is therefore a need for an improved optical coupler over conventional techniques, allowing optical coupling between a variety of optical conductors, with increased obtainable energetic efficiency, reducing loss of signal strength between coupled products, and being more robust to damage of the coupler.